Methods for Emergency Mine Communications Using Acoustic Waves Time Synchronization and Digital Signal Processing

ABSTRACT

Acoustic waves are transmitted bidirectionally through-the-earth in a simple robust architecture in combination with signal-to-roise reduction techniques which ensure the ability to communicate using simple tools available in a mine. An enhanced version includes a personal electronics device designed to be carried by a miner to automatically decode acoustically transmitted text messages.

FIELD OF INVENTION

The invention disclosed herein is most closely allied with art found in USPC classes 367, 340, and 702. The invention utilizes acoustic waves through-the-earth to establish emergency communications in an underground mine. The signal-to-noise (S/N) is greatly improved by establishing a procedure to synchronize the time window of communications, encoding of data, and by utilizing digital signal processing techniques to exploit a priori knowledge of the time window and data encoding.

BACKGROUND OF THE INVENTION

Emergency communications between miners and rescuers remains a significant problem. While there are well established systems for normal communications within a mine and between the miners and above ground, in most emergencies the infrastructure can be damaged—leaving miners at a loss for communications. Recent mine emergencies, legislation, regulations, and prior art illustrate the state-of-the-art.

Notable Mine Emergencies

Quecreek

At approximately 9 PM on Jul. 24, 2002 an abandoned mine was accidentally breached by miners in the Quecreek mine, located in Somerset County, Pennsylvania. According to a story in Wikipedia, 9 miners were cut off from the mine entrance by water that flooded the mine. All communications with the miners was lost shortly after the accident. Water quickly flooded the mine, and by 9:15 AM on July 25, water was exiting the portals. Rescuers decided that if the miners were alive, the best guess was to drill a hole into one of the higher locations of the mine, in hopes that the miners had found an air pocket at a high point. The location was approximately 3000 feet from the entrance shaft and under 240 feet of earth.

According to a Jan. 24, 2003 article by John Stenmark in Point of Beginning (a surveying trade magazine) titled Saved by Surveying, Satellite and skill; engineering technician Bob Long, established the location of the desired drilling point by 1:25 AM July 25, after only 90 minutes on site. This rapid and accurate location was made possible by modern surveying methods—including GPS. Based on the location surveyed by Long, a 6.5 inch borehole was started at 2:05 AM and broke through into the mine at 5:06 AM. Rescue workers tapped on the shaft and received 3 strong bangs in response.

With confirmation that there were survivors, air was pumped in to create a higher pressure in the space to push the rising water out. Despite the attempt to pressurize the space, the water continued to rise and water covered the airshaft—which prevented the miners from signaling by tapping on the airshaft. By noon, the miners were about 300 feet from the airshaft. They resorted to banging on the rock ceiling with a hammer, which was detected by seismic equipment brought in by Federal mining officials.

A 30 inch hole was begun at 6:45 PM on July 25. After breaking a drill bit, a 26 inch bit broke through at 10:16 PM on July 27. Rescuers signaled to the miners by tapping on the drill shaft with a hammer, and a faint response was heard. All 9 miners were removed from the mine by 2:45 AM on July 28. All of the miners suffered from the cold wet conditions, with body temperatures between 92.5 and 96.8.

Ironically, the root cause of the Quecreek mine disaster was an undated and uncertified map of the abandoned mine, which was abandoned in 1963. One of the factors that made it possible to locate the drilling locations for the mine in which the miners were trapped was modern surveying techniques and GPS. The Quecreek disaster is the first known case of using GPS equipment to quickly and accurately locate a rescue point from above ground. Engineering technician Long earned a degree of notoriety for his work, but the Quecreek mine disaster brought attention to the mine operators that accurate mine surveys tied to GPS coordinates could be very useful in an emergency situation. It is now accepted that locations in a mine can accurately, and rapidly, be projected to above ground coordinates.

Sago

At 6:30 AM on Jan. 2, 2006 there was an explosion in the Sago Mine in Sago, W. Va. According to a story in Wikipedia; thirteen miners were trapped about 2 miles from the entrance to the mine and under about 280 feet of earth. Fumes and heavy smoke between the miners and the entrance prevented escape or entry by a rescue team. All communications were severed, so two 6.25 inch holes were drilled into the area considered to be the most likely shelter for the miners. The first hole was completed the morning of January 3, but no signs of life were detected. When conditions cleared, a rescue team made its way in through the entrance in the early morning of January 4, and found one survivor, Randal McCloy Jr., in critical condition. The other 12 miners had died from carbon monoxide poisoning. McCloy was removed at approximately 1:30 AM and treated for carbon monoxide poisoning, a collapsed lung, brain hemorraging, edema, muscle injury, faulty liver and heart function. Recovery required months of treatment and rehabilitation.

McCloy later recounted the story in an article in the Charleston Gazette on Apr. 28, 2006. He stated they;

-   -   attempted to signal our location to the surface by beating on         mine bolts and plates. We found a sledgehammer, and for a long         time we took turns pounding away. We had to take off the         rescuers in order to hammer as hard as we could. This effort         caused us to breathe much harder. We never heard a responsive         blast or shot from the surface.

The futile attempts to signal their location had gone undetected topside. Had the rescue team known for sure that there were survivors, and the approximate location, perhaps a larger shaft could have been drilled in time to rescue more of the miners or McCloy could have been rescued earlier and spared some of his injuries.

Crandall Canyon

At 2:48 AM on Aug. 6, 2007 there was a mine collapse at Crandall Canyon Mine in Emery County, Utah. According to a story in Wikipedia; six miners were believed to be approximately 3.4 miles from the mine entrance and 1500 feet undergroung. All communications were severed, so a series of six holes were drilled in the most likely locations for survivors to shelter. The first was a 2.5 inch hole that broke through at 9:47 PM on August 9. The sixth was completed on August 23. No life was detected through any of the holes.

On August 16, a second collapse killed three rescue workers and injured six others. After the sixth hole was completed with negative results, the rescue was terminated and the bodies of the miners were never recovered.

Had there been better communications, and survivors, perhaps the first hole could have been drilled in the actual location. Had there been no survivors, and high probability that the absence of communications indicated no survivors, the rescue team would not have been put in as much danger. Perhaps the loss of life and injury suffered by the rescuers could have been avoided.

San José

On Aug. 5, 2010, there was a cave-in at the San José mine in the Atacama Desert near Copiapó, Chile. According to a story in Wikipedia; 33 miners were trapped approximately 3 miles from the entrance and 2,300 feet below ground. There was a second collapse on August 7, which halted attempts to rescue the miners through the existing mine shafts. All communications were severed, so holes were drilled in the most likely locations for the miners to shelter.

The first broke through on August 19, but no signs of life were detected. The eighth hole broke through on August 22. The eighth drill located the miners after 17 days. All 33 miners were rescued on Oct. 13, 2010.

Had there been better communications, perhaps the eighth hole location could have been drilled first, and the miners could have been spared 17 days of uncertainty as to whether or not they were going to be rescued.

US Mine Safety Legislation and Regulations

In the United States, mine safety is governed by the Federal Mine Safety and Health Act of 1977. This legislation created the Mine Safety and Health Administration (MSHA) under the Secretary of Labor. As will be explained in more detail hereinbelow, the National Institute for Occupational Safety and Health (NIOSH) is administered under the Secretary of Health and Human Services and is also responsible for aspects of mine safety.

Following the Sago Mine accident, The Federal Mine Safety and Health Act of 1977 was amended by the Mine Improvement and New Emergency Response Act of 2006, or the “MINER Act”. Under the amendment PL 109-236 (5 2803) Jun. 15, 2006; Title 30 of the United States Code, 30 U.S.C., which is administered by the MSHA, was amended to add POST-ACCIDENT COMMUNICATIONS under 30 U.S.C. 876(b)(2)(E)(i), which states;

-   (i) POST-ACCIDENT COMMUNICATIONS The plan shall provide for     redundant means of communication with the surface for persons     underground, such as secondary telephone or equivalent two-way     communication.

The act also added 30 U.S.C. 876(b)(2)(F)(ii);

-   (ii) POST ACCIDENT COMMUNICATIONS—Not later than 3 years after the     date of enactment on the Mine Improvement and New Emergency Response     Act of 2006, a plan shall, to be approved, provide for post accident     communications between underground and surface personnel via     wireless two-way medium, and provide for an electronic tracking     system permitting surface personnel to determine the location of any     persons trapped underground or set forth within the plan the reasons     such provisions can not be adopted, the plan shall also set forth     the operator's alternative means of compliance. Such alternative     shall approximate, as closely as possible, the degree of function     utility and safety protection provided by wireless two-way medium     and tracking system referred to in this subpart.

The act also added 29 U.S.C. 671(h), which is administered by NIOSH (the Institute), to create the Office of Mine Safety and Health;

(h) OFFICE OF MINE SAFETY AND HEALTH—

-   -   (1) IN GENERAL—There shall be permanently established under the         Institute an Office of Mine Safety and Health which shall be         administered by an Assistant Director to be appointed by the         Director.     -   (2) PURPOSE—The purpose of the Office is to enhance the         development of new mine safety technology and technological         applications and to expedite the commercial availability and         implementation of such technology in mining environments.

(3) FUNCTIONS—In addition to all purposes and authorities provided for under this section, the Office of Mine Safety and Health shall be responsible for research, development, and testing of new technologies and equipment designed to enhance mine safety and health. To carry out such functions of the Institute, acting through the Office, shall have authority to— . . . .

Under 29 U.S.C. 671(h), the Office of Mine Safety and Health (under NIOSH and the Secretary of Health and Human Services), is providing technology assistance to MSHA (under the Secretary of Labor), to implement the MINER Act mandate for POST ACCIDENT COMMUNICATIONS under 30 U.S.C. 876(b)(2)(F)(ii).

The NIOSH web site publishes updates on projects and activities under its MINER ACT of 2006 page. A report of Projects and Activities, updated Jan. 10, 2011, list 20 Contracts and Grants awarded for emergency communications. For example:

The National Institute of Standards and Technology has a contract to develop modeling and simulation tools to evaluate the performance of underground mine communications networks.

Helium Networks has a contract to design and develop a tool for mapping wireless coverage in underground coal mines.

Foster-Miller, Inc. has a contract to develop guidelines for safe management of electrical equipment and systems during a mine emergency or other abnormal circumstance.

CSIRO has a contract to evaluate the feasibility of a fiber optic sprinkler head emergency communications system for use in underground mines.

Pillar Innovations LLC has a contract to investigate techniques to increase the survivability of leaky feeder communications systems.

Rajant Corp. has a contract to develop a media converter device to interface wireless handheld radios with leaky feeder communication systems.

Ultra Electronics Canada Defense, Inc. has a contract to fabricate and test through-the-earth communication devices in an underground mine.

DISA Joint Spectrum Center has a contract to develop guidelines for safe and efficient use of the limited radio spectrum in underground mines.

CERMUSA has a contract to test efficacy of deploying a vehicular satellite communication system for use during mine rescue and other emergency events.

CONSPEC Controls, Inc. has a contract to develop a robust intrinsically safe system that will incorporate atmospheric monitoring, two-way communications and miner tracking, all in a single network.

URS Group, Inc. has a contract to determine appropriate and practical risk analysis techniques for assessing and mitigating basic types of possible stored energy sources.

Foster-Miller, Inc. has a contract to develop recommendations for battery selection, use, and charging in underground coal mines.

U.S. Army CERDEC has a contract to adapt a subterranean wireless communications system for use in underground mines.

Foundation Telecom, Inc. has a contract to develop a universal signal evaluation package for 75 and 150 MHz bands, adapt a passive magnetic amplifier for 900 MHz, and investigate energy harvesting technologies.

Foster-Miller, Inc. has a contract to develop a method to evaluate the reliability and survivability of underground communication, tracking, and atmospheric monitoring systems.

Alertek, LLC has a contract to develop a battery-powered, through-the-earth wireless voice communication system for overburdens of up to 600 feet.

Lockheed Martin Corp. has a contract to develop and demonstrate a two-way, through-the-earth communication system for mines.

Stolar Research, Inc. has a contract to design, fabricate, and test a prototype, two-way, through-the-earth emergency communication system.

E-Spectrum Technologies has a contract to adapt an existing ULF through-the-earth system for communication and tracking of underground miners.

L-3 Global Security & Engineering Solutions has a contract to design, install, and evaluate a wireless mesh communication and tracking network in an underground coal mine.

An excellent review of the state-of-the-art was published by CANMET Mining and Mineral Sciences Lab (MMSL), which is an agency of Natural Resources Canada. Report CANMET-MMSL 09-004(TR), Summary Study of Underground Communications Technologies, May 2009, is incorporated by reference herein. The 113 page report covers inter alia; Very Low Frequency Through-the-earth Communications, MF or Medium Frequency Communications, VHF Leaky Feeder Communications, UHF Leaky Feeder Communications, Distributed Antenna Systems, Wi-Fi Communications, Mesh Networks, and Ultra Wide Band (UWB) Communications.

Despite the mandate by Congress to prove a plan for Post-Accident Communications by Jun. 15, 2009, the technology has not been successfully demonstrated. MSHA PROGRAM POLICY LETTER NO. P09-V-01, dated Jan. 16, 2009, on the SUBJECT: Guidance for Compliance with Post-Accident Two-Way Communications and Electronic Tracking Requirements of the Mine Improvement and New Emergency Response Act (MINER Act), which is incorporated by reference herein, states;

-   -   However, because fully wireless communications technology is not         sufficiently developed at this time, nor is it likely to be         technologically feasible by Jun. 15, 2009, this guidance         addresses acceptable alternatives to fully wireless         communication systems.         The expiration date of P09-V-01 was Mar. 31, 2011.

MSHA PROGRAM POLICY LETTER NO. P11-V-13, dated Apr. 28, 2011, on the SUBJECT: Guidance for Compliance with Post-Accident Two-Way Communications and Electronic Tracking Requirements of the Mine Improvement and New Emergency Response Act (MINER Act), which is incorporated by reference herein, updated P09-V-01 and states;

-   -   However, because fully wireless communications technology is not         sufficiently developed at this time to permit use throughout the         industry, this guidance addresses acceptable alternatives to         fully wireless communication systems. New ERPs [Emergency         Response Plans] and revisions to existing ERPs should provide         for alternatives to fully wireless communication systems.         The expiration date of P11-V-13 is Mar. 31, 2013. A review of         the NIOSH and MSHA web sites does not indicate that there has         been significant progress, so it is expected that the policy         will be extended beyond Mar. 31, 2013.

Acoustic Communications

It will be understood that the terms acoustic, seismic, or sound, as used herein, will include all mechanical vibrations and is not limited to the human aural sensitivity frequency of 20-20 000 Hz. For example, seismic vibrations—which may be referred to as acoustic or sound—may extend to fractions of 1 Hz. A review of the NIOSH Contracts and Grants, and the CANMET-MMSL report reveals a conspicuous absence of research and products using acoustic communications through-the-earth. In fact, there are no contracts for acoustic communications listed, and there is no mention of the technology in the CANMET-MMSL report. Yet in the accident reports listed hereinabove, the most basic and simplest communication was transmitted acoustically.

Prior Art

The prior art discloses a number of ideas that have been proposed for seismic communications.

U.S. Pat. No. 3,273,112 to Hobson, incorporated by reference herein, discloses a tuned seismic wave communication system for military communications which is invulnerable to nuclear attack and difficult to jam. Hobson discloses a variety of seismic transmitters and receivers, as well as techniques for coupling to a rock strata—such as using a closed end well casing filled with water to connect the surface with an underground rock strata. Hobson also discloses encoding schemes such as Morse code by switching a fixed frequency vibration on and off, and frequency modulation.

U.S. Pat. No. 3,715,664 to Ikrath, incorporated by reference herein, discloses a method of repeating RF-borne signal across an earth barrier, in which in one embodiment a seismic-acoustic transmitter is used to transmit a signal through a mountain to a receiver on the other side under military conditions, e.g., conditions that would not be conducive to setting up a radio tower on top of the mountain. The preferred band is between 70 and 100 Hz, with 78-83 Hz for soft earth and around 80 Hz for hard rock. For soft earth, 10 Watt transducers are recommended and for hard rock around 60 Watts is preferred. Results are cited for successful commuication in a mine from the surface through 1600 feet vertically, and greater than 0.5 miles across hilly terrain.

U.S. Pat. No. 7,307,915 to Kimball, incorporated by reference herein, discloses a seismic modem for the transmission of data. Problems of transmission through an inhomogeneous medium, reflections, refraction, dispersion, and other sources of waveform distortion are addressed by preceding communications by a training sequence of waves which are used to estimate transmission parameters. These parameters are then applied to the communications signals to improve the signal-to-noise ratio and extract the signals from background noise. One preferred embodiment uses a transducer built into footwear, and uses the weight of the wearer to improve coupling of the transducer to the ground.

U.S. Pat. No. 7,843,768 to Squire et al., incorporated by reference herein, discloses a system for communicating location of survivors in mine emergencies by transmitting seismic waves of pre-selected frequencies. The signal-to-noise ratio is improved by preferably using frequencies between 40 and 85 Hz to minimize attenuation, using between 100 and 1000 Watts to drive a transducer, using a high Q bandpass filter at the receiver, and using Fourier filtering to extract the pre-selected frequencies.

US 2009/0207693 to Schuster, incorporated by reference herein, discloses a seismic location and communication system which uses a library of pre-recorded reference signals produced by underground seismic generators detected by an array of detectors. In an emergency, the signal from a seismic generator is cross-correlated with the library of pre-recorded reference signals to detect the signal, which identifies the signature of the seismic generator—and thus the location. The seismic generators are permanently installed in a mine at selected locations, and may be activated by manually striking with a hammer. The signal-to-noise ratio is greatly improved by the array of receivers and cross-correlation operation, which allows the signals to be extracted from the background noise. Messages may be transmitted by coding the signal generation using coding techniques such as Morse code.

US 2009/0316530 to Bunyard et al., incorporated by reference herein, discloses a bi-directional seismic communication system in which seismic communication stations are established at safety shelters in a mine. Metal rods are anchored into the rock to enhance seismic coupling to transducers or manually striking with a hammer. Various encoding schemes are disclosed including encoding schemes for Morse code and tokens to optimize efficiency. Experimental results are shown for various signal sources detected at 1508 feet—including a sledgehammer showing a signal-to-noise ratio of 35.9 dB.

US 2010/0135116 to Bis et al., incorporated by reference herein, discloses a miner acoustic communication and locating system in which a pneumatically driven mechanical piston device automatically generates a continuous acoustic wave to signal the location of trapped miners. The device is powered by solid propellant gas generators, similar to those used to inflate automobile airbags. An array of detectors above ground is used to triangulate the location of the source.

US 2010/0322034 to Yang et al., incorporated by reference herein, discloses acoustic communication and locating devices for underground mines in which miners communicate above ground using battery powered portable miner signal units to transmit modulated acoustic signals to a base station above ground. The base station has an array of transceivers to receive and transmit signals from/to the miners. The miner signal units include an impact actuator, a control system, and a user interface. The miner enters a message via the user interface which is then encoded by the control system to produce modulated seismic signals which are received by the base station via the array of transceivers. A library of baseline signals, which are updated periodically prior to an emergency, are used to correlate with the received signal to improve the signal-to-noise ratio, triangulate the location of the signal source, and resolve simultaneous miner signal unit signals.

While all of the patents cited hereinabove have merits, they all suffer from shortcommings. For example, some require extensive preparation and prepositioning of heavy equipment. In an actual emergency, a miner may not be able to get to a predefined location. With more sophisticated equipment, there is an ongoing problem of maintenance to ensure that it will actually work in the rare event it is actually called upon.

A number of problems are obviated by the disclosed invention.

BRIEF SUMMARY OF THE INVENTION

Methods are disclosed for implementing an emergency communications system in compliance with the Mine Improvement and New Emergency Response Act of 2006 (MINER Act), and Mine Safety and Health Administration regulations pertaining to Post-Accident Communications.

Acoustic waves are transmitted bidirectionally through-the-earth in a simple robust architecture in combination with signal-to-roise reduction techniques which ensure the ability to communicate using simple tools available in a mine. An enhanced version includes a personal electronics device designed to be carried by each miner to increase the data rate.

Algorithms for determining time windows for communications are established in order to enhance the signal-to-noise ratio and coordinate the efforts of miners and rescue personnel. There is also a psychological benefit for the miners since they can rest during the breaks between windows and not have to be constantly on alert. Data is encoded in accordance with predetermined encoding schemes in order to ensure proper interpretation of a signal. Digital signal processing is employed to further improve the signal-to-noise. An engineering study is performed to identify sources of noise, steps for noise remeadiation, transducer locations, and ensure reliable communications with all parts of a mine. An Emergency Response Plan is established and tested.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, reference will be made to the accompanying drawings, which serve to illustrate various embodiments of the invention by way of example.

FIG. 1A shows elements of a preferred up channel communications system.

FIG. 1B shows elements of a preferred down channel communications system.

FIG. 2 shows a sectional view of the underground and surface architecture of an embodiment of the invention.

FIG. 3 shows steps for up channel communications.

FIG. 4 shows steps for down channel communications.

FIG. 5 shows steps for an Emergence Response Plan in compliance with Mine Safety and Health Administration regulations.

DETAILED DESCRIPTION OF THE INVENTION

What is needed is a robust, yet simple, method for communicating through-the-earth by acoustically generated waves. The prior art described hereinabove may be useful in certain circumstances, but for the most part it all relies on equipment that is too complicated, expensive, or heavy, for reliable emergency use. Moreover, no known prior art teaches improvement of signal-to-noise ratio (S/N) by the combination of techniques disclosed herein.

As shown in FIG. 1A an ideal method at the underground end 100 would require very little or no electrical power 105—preferably at most no more than the power to operate a flashlight or a cell phone. Be inexpensive and suitable for carrying untethered by each miner at all times 110, e.g., a watch or personal electronic device. A signal could preferably be generated by manual effort 115 and implemented with a variety of tools adapted from items which would already be available in a mine 120. Require very little or no maintenance of underground equipment 125. It could be implemented from anywhere in the mine 130, i.e., would not require prepositioning equipment at predefined locations which must be maintained and may not be accessable in an emergency anyway. Personnel training should be minimal and easy to understand 135. Emphasis should be placed on reliability over bandwidth 140, i.e., it is more important to signal that there are survivors than it is to carry on high bandwidth voice communication—particularly for the channel communicating from the miner to rescuers. The system should be robust enough that the absence of a signal could be reasonably used to conclude that there are no survivors, and thus avoid exposing rescuers to unnecessary risks. Emergency Response Plans (ERPs) prepared around the method must meet the requirements of MSHA PROGRAM POLICY LETTER NO. P11-V-13 and other regulations 145.

As shown in FIG. 1B the above ground equipment 160 could be much more sophisticated, but ideally would be economical enough to have necessary field components, such as transducers and communications equipment, prepositioned at every working mine for quick response 165. Other components, such as digital signal processing computing equipment, communications equipment, and technical experts, could be remotely located at a central laboratory 170. This would avoid duplication of equipment and allow some of the expenses to be shared by the industry. The bandwidth from the rescuers to the miners may be larger 175, which may be used to minimize the necessary response from the miner to simply yes and no replies to a series of questions.

This invention discloses such a method for emergency mine communications. While the invention will be illustrated by examples in the most basic and robust forms, it will be recognized that enhanced embodiments would be practical for established shelter facilities that may include more resources.

From the CANMET-MMSL 09-004(TR) report cited hereinabove, it is clear that none of the existing systems meet the criteria. It will be shown that an acoustic system in combination with engineering, planning, and signal processing, is capable of meeting the criteria.

The fundamental problem is a matter of S/N. A miner randomly pounding on a mine wall or ceiling with a sledge hammer will probably go undetected above ground for a number of reasons.

Even with mine activities stopped in an emergency, there will be a lot of equipment running. For example, there will probably be ventilation systems, pumping systems, and other necessary utility systems operating. There may also be drilling operations, emergency power generators, earth moving equipment, chainsaws, construction, etc., making preparations for a rescue attempt. It may not be practical to shut all equipment down to “listen” for a faint sound. It will be understood that the term “listen” as used herein is used in the general sense of detecting mechanical vibrations of all acoustic frequencies and not limited to human aural frequencies or human senses. Moreover, at present there is no protocol as to when to listen for signaling from underground. In general, it would be hit-and-miss to detect an underground signal. Example protocols which enhance the probability of a faint signal being detected will be disclosed which serve to illustrate the spirit and scope of the invention.

There are signal processing techniques that can be adapted to greatly improve the S/N. In general, the techniques may be different for communication from the miner(s) to the rescuers, which will hereinafter be referred to as the up channel; and for communication from the rescuers to the miner(s), which will hereinafter be referred to as the down channel.

Fundamentals of Geophysics

A brief review of the non-patent literature and published patent literature prior art will be useful background information which will help in understanding the spirit and scope of the invention and develop the theoretical basis of the physics involved.

In 1920, J. C. Karcher was working on methods of sound ranging to detect artillery for the United States Bureau of Standards (now National Institute of Standards and Technology), as describe in an article in Wikopedia. Working with professors at the University of Oklahoma, he thought that the same methods could be used for seismic exploration. They formed a company in 1920, which eventually became Texas Instruments, to demonstrate the technique. In a parallel path, Ludger Mintrop was developing similar ideas in Germany, which led to U.S. Pat. No. 1,451,080, and U.S. Pat. No. 1,599,538, both of which are incorporated by reference herein.

Since the earliest seismic reflection field test by J. C. Karcher on Jun. 4, 1921 in central Oklahoma, which was commemorated by the Geophysical Society of Oklahoma City on the 50 year anniversary by placing a monument on the site—as noted in Introduction to Geophysical Prospecting, Dorbin, (Dobrin) p. 10-11, incorporated by reference herein—there have been technical advances made in geophysics, seismic surveying (or seismic exploration), and drilling which can be applied to the problem of acoustic communications. For example, it is common practice in the drilling art to communicate with a drill string via acoustic communications for measurement while drilling (MWD), and logging while drilling (LWD). It is also common practice to introduce high power modulated seismic waves into the earth for seismic surveying. Digital signal processing techniques are also well known in the art.

Turning first to the non-patent literature; Introduction to Applied Geophysics, Burger, Sheehan, and Jones (Burger) is a modern introduction to geophysics. Chapter 2, which is incorporated by reference herein, covers Seismic Exploration: Fundamental Considerations. In section 2.1.3, Burger describes P-waves, S-waves, and surface waves in detail which will not be repeated herein. Table 2.2 tabulates representative P-wave velocities for some common materials. For example; air—331.5 m/s, soil—250-600 m/s, clay—1100-2500 m/s, saturated sandstone—800-2200 m/s, limestone—2000-6000 m/s. The dispersion which generally causes sound to travel slower through the upper level of the earth presents problems in seismic exploration which will be addressed by signal processing techniques discussed hereinbelow. In section 2.3.1, Burger describes Spherical Spreading and points out that for a point source, the energy falls off as 1/r² where r is the radial distance from the point source, whereas the amplitude of a vibration is proportional to the square root of the energy, and thus the amplitude falls off as 1/r. In section 2.3.2, Burger describes Absorption, and it can be shown that the combined reduction in elastic energy can be expressed as

$\begin{matrix} {I = {{I_{0}\left( \frac{r_{0}}{r} \right)}^{2}^{- {qr}}}} & (1) \end{matrix}$

where I is the energy intensity at a distance r from a point source, I₀ is the energy intensity at a distance r₀, q is the absorption coefficient in dB/λ, and λ is the wavelength, i.e., the speed of sound/frequency. Section 2-6 of Dobrin (cited hereinabove), which is incorporated be reference herein, explains how the absorption coefficient can be related to the quality factor Q, which is well known in the art.

As explained in section 3.5 of An Introduction to Geophysical Exploration, Kearey and Brooks (Kearey), incorporated by reference herein;

-   -   Over the range of frequencies utilized in seismic surveying the         absorption coefficient is normally assumed to be independent of         frequency. If the amount of absorption per wavelength is         constant, it follows that higher frequency waves attenuate more         rapidly than lower frequency waves as a function of time or         distance. To illustrate this point, consider two waves with         frequencies of 10 Hz and 100 Hz to propogate through rock in         which ν_(p)=2.0 km s⁻¹ and α=0.5 dBA⁻¹. The 100 Hz wave (λ=20 m)         will be attenuated due to absorption by 5 db over a distance of         200 m, whereas the 10 Hz wave (λ=200 m) will be attenuated by         only 0.5 dB over the same distance. The shape of a seismic pulse         with a broad frequency content therefore changes continuously         during propagation due to the progressive loss of the higher         frequencies. In general, the effect of absorption is to produce         a progressive lengthening of the seismic pulse.

The implications of the fact that absorption per wavelength is constant is further explained by Burger in section 2.3.2, which points out that the earth is a low-pass filter, and slower S-waves are attenuated more than faster P-waves. Note that as shown in the example, the absorption of low frequency sound waves through rock is orders of magnitude less than the absorption of electromagnetic waves, which makes acoustic communications fundamentally practical where electromagnetic communications is not.

Burger covers Energy Sources for producing vibrations, in section 2.4. Weight drop, explosive, and vibratory sources are outlined in section 2.4.1, including the following description of hammer sources, which will be useful hereinbelow;

-   -   The simplest example of the weight drop is the hammer source. A         sledgehammer, usually 5.4 or 7.3 kg, is swung against a metal         plate to more efficiently couple the energy transfer. Obviously         the energy imparted is not great, and in the past a hammer         source was used only for very shallow investigations. The hammer         blow became a more useful source with the advent of relatively         inexpensive instruments that could sum ground motion for a         number of impacts and thus enhance the waveforms on records.         Under good conditions a hammer source routinely can detect the         overburden—bedrock interface at depths of 50 meters or slightly         more.

It should be noted that in the last sentence of the quote above, Burger is saying that under good conditions; a hammer can be used to generate a vibration that travels up to 50 meters through-the-earth, a fraction of the vibrational energy reflects off the overburden—bedrock interface and travels back 50 meters through-the-earth, and is detected with sufficient accuracy to determine the depth of the interface. Clearly it would be detectable for a distance much greater than 100 meters in a direct line, without reflecting a fraction of the energy, and traveling substantially through rock instead of exclusively through overburden.

In the previously cited US 2009/0316530, Bunyard et al. report a S/N level of 35.9 dB at a distance of 460 m for a sledge hammer strike. They estimate a S/N of 9 dB at 862 m. Recall that Quecreek was a depth of 73 m, Sago was 85 m, Crandall Canyon was 457 m and San José was 701 m. With signal processing, it is reasonable to detect a signal at levels much lower than a S/N of 9 dB.

In FIG. 2.26 of section 2.4.1, Burger shows a plot of energy vs frequency for an 8-gage shotgun, a 12-gage shotgun, a 75 kg weight drop, and a 7.3 kg hammer. All sources show similar characteristics of maximum energy between 200 and 400 Hz. As explained hereinabove, the higher frequencies will be attenuated more as the vibrations propogate, so the shape of the curves would change with distance from the source.

Turning now to the published patent literature, U.S. Pat. No. 2,688,124 ('124) to Doty and Crawford, incorporated by reference herein, discloses correlation techniques developed by Continental Oil Company which uses vibrators to produce semi-continuous signals instead of impulse functions produced by explosions. Known as the Vibroseis® system, signals are produced by vibrators anchored to the earth which produce 10 to 20 ton forces—much like a programmable function generator for seismic waves.

Doty and Crawford used a vibrator comprising counter-rotating off-center weights to produce P-waves directed into the earth. The vibrator was driven by a motor which was swept from 20 Hz to 80 Hz over a 4 second sweep, and then repeated as needed to gather sufficient data. A transducer, or array of transducers, were placed away from the vibrator to detect reflected vibrations. A “counterpart” of the transmitted signal was detected near the vibrator. The counterpart and reflected signals were recorded in separate channels on a magnetic drum. The time delay was determined in post processing by cross-correlating the two signals.

The Continental Oil Company '124 patent to Doty and Crawford, filed in 1953, is considered to be a pioneering patent in the field, the historical development of which was published by a member of the development team in Vibroseis, Nigel A. Anstey, Prentice Hall, 1991. A key factor in the development, as explained by Anstey, was a short course Doty attended that explained chirped radar, developed during WW II, after it became de-classified. Doty realized he could greatly improve the coupling efficiency over explosive charges, as used by Karcher in 1921, by driving lower frequency vibrations, which as explained hereinabove are attenuated much less than the higher frequencies produced by explosives. Moreover, he could avoid the problem of the absolute distance ambiguity by using a chirped signal—much like chirped radar.

Doty and Crawford et al. made additional improvements in the signal processing methods and apparatus in U.S. Pat. No. 2,808,577; U.S. Pat. No. 2,874,795; U.S. Pat. No. 2,910,134; U.S. Pat. No. 2,989,726; and U.S. Pat. No. 3,209,322; all of which are incorporated by reference herein.

U.S. Pat. No. 4,486,866 to Muir, incorporated by reference herein, discloses methods for seismic exploration using vibratory sources to inject coded vibrations into the earth and correlating received signals to improve S/N.

U.S. Pat. No. 4,866,680 to Scherbatskoy, incorporated by reference herein, discloses methods for improving S/N for MWD using telemetry data transmitted through drilling fluid.

U.S. Pat. No. 5,850,369 to Rorden et al., incorporated by reference herein, discloses a system in which a chirp and two tones are transmitted acoustically through-the-earth to synchronize downhole and surface clocks.

U.S. Pat. No. 6,308,137 to Underhill et al., incorporated by reference herein, discloses methods for MWD wherein acoustic communication is on a schedule which is known by the downhole equipment. Of course this require time synchronization between the surface and downhole equipment, but it improves S/N and conserves power.

U.S. Pat. No. 6,400,646 to Shah et al., incorporated by reference herein, discloses a system in which the downhole and surface clocks are synchronized by acoustic communications with millisecond accuracy.

U.S. Pat. No. 6,530,263 to Chana, incorporated by reference herein, discloses a method for detecting leaks in a pipeline by using an array of loggers which record data for post-processing by correlation.

U.S. Pat. No. 6,584,406 to Harmon et al., incorporated by reference herein, discloses a seismic communications system for signaling a device in a borehole—such as an oil well. A control system generates coded signals which are transmitted through-the-earth to devices near the bottom of the well by modulating a Vibroseis® transducer at the surface. Geo-phones at the surface monitor the Vibroseis® generated signal, and provide feedback to the control system to ensure proper encoding is generated. A receiver in the well decodes the seismic signal using a microprocessor and digital signal processing. The decoded seismic signal is then used to instruct devices in the well. In one preferred embodiment, instructions are transmitted to fire a gun which perforates the well casing to produce the well.

US 2007/0250269 to Wei et al., incorporated by reference herein, discloses an apparatus for generating seismic signals.

While the field of geophysics is well developed for seismic exploration, there are no known adaptations of the technology to emergency communications—which this invention obviates.

Solution to the Problem

Having developed a background in the physics of the subject matter and the prior art, it will now be shown how acoustic waves can be used in new ways for emergency mine communications.

Consider the following pedagogical example of a method for emergency communications, which will later be modified into practical embodiments. In '124 to Doty and Crawford, as described hereinabove, the objective is to measure the time delay between a signal source and detector for seismic waves reflected by underground discontinuities, i.e., changes in rock formations. One complication addressed by '124, and their subsequent patents, is the fact that in addition to the reflected waves of interest, there are also seismic waves transmitted directly between the source and detector. For seismic exploration, this is considered to be noise.

For the problem at hand, it is clear that this noise can be considered a communication channel between the source and detector which can be detected by cross-correlating the detected signal with the “counterpart” signal generated near the source. Of course both the source and detector are located on the surface for seismic exploration purposes.

Suppose the architecture was modified and the source was located in a mine, and the detector was located on the surface in relative proximity to the mine. It will be recognized that for pedagogical purposes the mechanism would be symmetric and the locations of the source and detector could be reversed with the same results. For this purpose, also assume for the moment that there is an electrical signal between the source and the detector to transmit the “counterpart” signal to the detector end. It is clear that the source could send signals through-the-earth to the detector which could be decoded with the assistance of a computer to record and cross-correlate the “counterpart” and detected signals, i.e., an up channel. It is also clear that the same system could be reversed to communicate a down channel.

Of course it would be absurd to go to such measures if there was wiring to connect the “counterpart” with the detector end, i.e., one would simply use the wiring for the counterpart for a communications system. In the absence of a counterpart signal—as one would expect in the case of an emergency—there are other techniques that will be shown to be useful to recover a seismic signal.

It would be highly unlikely that a miner would have a vibrator as used in '124 at his disposal in an emergency situation. While it would be possible to have vibrators driven by battery power, manual crank mechanisms, compressed air, mechanical wind-up mechanisms, weight driven mechanisms, or the like, positioned at staging areas; it would not meet the requirements for a robust emergency communications system 100 as outlined hereinabove.

Starting with a system as described in '124 without the counterpart signal and without the vibrator to generate seismic systems, a practical system 200 will be developed as shown in FIG. 2. A sectional view shows the ground level 205 and underground level including the mine wall 210, mine floor 215 and mine roof 220. Between the ground level 205 and the mine roof 220 is a layer of rock and overburden 225. A miner 230 strikes the mine wall 210 with an tool 235 such as a sledge hammer or the like. Transducers 240A, 240B, 240C, 240D are located at the ground level 205. A seismic vibration generator 245 has a support 250 in communication with the overburden 225. For the up channel communications, the miner 230 strikes the mine wall 210 with tool 235 producing vibrational waves 255 which radiate through 225 to the transducers 240A, 240B, 240C, and 240D. For the down channel communications, the seismic vibration generator 245 and support 250 produce vibrational waves 260 which radiate through 225 to the mine wall 210, mine floor 215 and mine roof 220. It will be understood that the number of transducers shown is merely exemplary and could be as few as one, or a large array.

As shown hereinabove, higher frequency vibrations are attenuated more than low frequency vibrations. It is well known in the art that explosions and hammers produce impulse functions which are rich in higher frequency components. Hence the vibrator generator taught in '124 is much more efficient than the explosion source used by Karcher. However, it is likely that the only seismic wave generator mechanism available to a miner would be manually striking the mine wall or ceiling with a hammer, battering ram, rock, or the like. For the up channel, the highly attenuated higher frequency components of such a mechanism could be compensated for by bringing more signal processing capabilities to bear on the receiver end.

Signal-To-Noise

The first method to improve S/N is to have an established protocol which sets time of day windows for the miner to signal via the up channel. This protocol which sets the time of day would be determined independently by the miners and the rescue personnel. The time of day could be triggered by an accident in the mine. Since each party would have significant uncertainty in the exact time of the accident, there is a quantization error which could be accounted for by also quantitizing the time of day windows to certain times, e.g., on the half hour mark. Assuming each party has a watch, there will be no ambiguity as to when the communications will start.

The benefits of limiting the search in the time domain to specific windows are manifold, and the use of time of day windows is a major feature of the invention. For example, if the rescuers know that the miner will be signaling in a specific time window, they could switch optional equipment off and cease activities during the specific window. All sensor signals could be recorded to facilitate more powerful post-processing—such as between an array of sensors, or using more powerful computers at laboratories in other parts of the world. The miners could conserve energy by avoiding needless hammering that would probably go undetected, and the anxiety of the miners would be reduced by limiting the windows of time they would need to be responsible for detecting a signal.

For example, a first example protocol would be for the miners to wait to communicate until at least 30 minutes after an accident, and commence communications on the next half hour mark for a window of two minutes. For example, if the accident occured at 8:15, the rescue team would know to be ready to record signals from 9:00-9:02, 9:30-9:32, 10:00-10:02, etc. This would give the rescuers time to get organized, and it would preserve the energy of the miners.

It will be recognized by those skilled in the art that there would be the possibility of a plurality of separated groups of miners, and there may be overlap in the signals between the groups. This will be addressed hereinbelow in the protocol for the down channel.

By using a known window in time, the chances of a signal being detected would be greatly increased. For example, by comparing sensor signals during the window to signals before and after the window, digital signal processing techniques could be used to detect differences between the window and other times. This is much like the technique used by a lock-in amplifier, which is well known in the art, where the signal is chopped in time. For example the power spectrum during the window may be significantly different than the power spectrum before and after the window. However, this alone may not be enough of an improvement in the S/N to have confidence that there were, or were not, surviving miners trapped below ground.

Another improvement in S/N could be achieved in one preferred embodiment by adding a protocol wherein the miners produced a signal at a known repetition rate, or repetition period, within a prescribed time window. Hereinafter the time of day window will be designated by a format of hour:minute:second. For example, if they used a sledge hammer to strike the mine wall every 10 seconds within a two minute window, the strikes would take place at 9:00:00, 9:00:10, 9:00:20, 9:00:30, . . . , 9:01:50. It will be understood by those skilled in the art that times used in examples to illustrate the spirit of the invention are merely examples and not limitations.

Such a signal would lend itself to powerful digital signal processing techniques such as cross-correlation, autocorrelation, Fourier analysis, power spectrum analysis, etc., which are well known to those skilled in the art. An event, such as a hammer strike, typically has a ringing signature with a decaying exponential amplitude envelope. The repetition period should be long enough to allow the previous exponential envelope to substantially decay to the background level, and to allow the person to prepare for the next strike without undue stress. In such a case, the signature of a single hammer strike from 9:00:00-9:00:10 would cross-correlate very well with the signature of the same single hammer strike from 9:00:10-9:00:20, as well as the other 10 second windows betweem 9:00:00 and 9:02:00. The power spectrum would show strong components around 0.1 Hz and higher harmonics. The presence, or absence, of strong cross-correlation between signatures in the 2 minute window vs before and after would give a much higher confidence as to the presence of survivors below.

A more common example will illustrate the idea of digital signal processing techniques. People hear a dripping faucet that repeats at regular intervals, even in the presence of background noise. This is due to subconsiously processing the signals in a way analogous to digital signal processing, i.e. it is not only the sound of the drip that is being used to detect the sound. It is not so much the sound or the signature of the drip, as it is the periodic pattern of the low level sound that makes it detectable, much like phase-lock loop detection. It is also clearly detectable if the drip stops or if the repetition rate changes, i.e., if it is modulated.

This is also seen in the cocktail party effect where it is common for people to converse in the presence of high background noise. Just as a person can tune to a particular voice or enhance understanding by also watching the speakers lips or body language, digital signal processing techniques can enhance a signal if the model for the signal is known.

In cases where signals are know to be periodic, the S/N is improved by stacking the signals, or performing an ensemble average. For example, if a signal s is sampled j times over a period of the signal, and the signal is repeated k times, an averaged signal S can be constructed that reduces the noise, where

$\begin{matrix} {{S(i)} = {\sum\limits_{n = 0}^{k - 1}{s\left( {i + {nj}} \right)}}} & (2) \end{matrix}$

for i=1 to j. For example, if the signal s is sampled at 10 kHz, and repeated every 10 seconds, j=10⁴ samples for a signature. If the signature is repeated every 10⁴ samples

S(i)=s(i)+s(i+10⁴)+s(i+2×10⁴)+s(i+3×10⁴) . . . s(i+(k−1)×10⁴)   (3)

In such a case, the signatures will be added, while random noise will average to zero. This process will significantly improve the S/N.

Most background vibrational noise will be due to operating machinery. For electrical equipment, large machines may be driven by synchronous motors, which are known in the art to be useful for improving the power factor (and thus reduce electrical power expense) for the electrical distribution system. Such machines will be highly repeatable at the power line frequency, or integer sub frequencies for large machines. In the US, electric power is distributed by a large power grid at 60.0 Hz. Due to the large capacity of the grid, the frequency is held very stable over short term periods as a result of the fact that all of the generators and motors on the grid are connected and operate together, i.e., the angular momentum of all of the rotating equipment on the grid acts as a low pass filter. A large compressor with a four pole synchronous motor will be mechanically driven at 30.0 Hz with little variation in vibrations for a constant load.

For example, oscilloscopes usually have provisions for a “line” trigger capability. When triggering on “line”, electrical noise that is synchronized with the AC power source, such as noise generated by fluorescent lights, is stationary on the display. This makes other signals detectable in the presence of noise which may otherwise totally hide the signal. The same technique can be used to remove repeatable, periodic, steady state, vibrational noise in a transducer signal.

An ensemble average of steady state signals, triggered on “line”, prior to a communications window could be subtracted for the detected signal during the communications window to greatly improve the S/N. Likewise, an ensemble average immediately after the communications window could also be used to improve the S/N in post-processing of the data. If there are dead times within the window, such as time allowances for the miner to prepare for the next hammer strike, the ensemble average could be conducted during the dead time—which would be closer to the actual conditions immediately prior or subsequent to the active portion of the window.

It will be recognized by those skilled in the art that heavy machinery may be automatically unloaded during the communications window to further reduce background noise. For example, a large compressor, chiller, blower, or the like, may simply freewheel during the communications window, and quickly switch back into service after the window. It will be recognized that unloading a compressor could be achieved by simply modifying the control system that normally unloades the compressor when the receiver has reached the upper pressure set point. Unloading a chiller could be achieved by simply modifying the control system that normally unloades the chiller when the desired temperature is reached. Unloading a blower could be achieved by closing dampers, thus reducing the volume of air forced through the ducts. This would avoid having to restart heavy equipment, which can require significant effort and danger of equipment failure.

Non-synchronous AC electric motors and Diesel driven engines are still highly periodic—although not as periodic as synchronous motors—and background vibrations may be reduced somewhat using the same technique. For example, a trigger signal may be generated by a transducer mounted on the crankshaft of a large diesel pump to remove periodic, steady state, noise due to the pump. Non-synchronous AC electric motors operate by what is known in the art as slip, i.e., the motor rotor rotates at a slightly lower frequency than the distribution frequency divided by an interger, e.g., 58, 25, 12, etc. Hz. The amount of slip is determined by the load, and can change dynamically based on the load. In addition to not being synchronized with the power distribution, they are also not synchronized with each other. This can produce beat frequencies in the vibrations. By including trigger signals from all large motors, further noise subtraction is possible. For example, the amplitude and phase of a signal synchronous with the trigger signal can be determined using techniques which are well known in the art, such as Fourier analysis, lock-in amplifiers, etc. Each signal component can then be removed in a post processing operation. Fortunately, most large motors are synchronous, and most DC motors would not be used for essential services.

In addition to giving priority to using synchronous motors for essential services that could not easily be switch off in an emergency, a master control system could be designed that would easily unload essential motors for short periods of time corresponding to the communications windows. It will be recognized that loading and unloading heavy machinery may need to be coordinated to avoid large power surges, which could trip overload protection relays. Plans could also be developed for reducing vibrations from such sources as railroad and roadway traffic for short periods of time in order to facillate acoustic communications.

In preparation for a mine emergency, an engineering study could be conducted to determine the requirements for a particular mine in order to reduce the background seismic vibrations to an acceptable level for that particular mine. For example, requirements for a particular mine may depend on such things as soil and rock types and the absorption coefficients, maximum depth of the mine, distribution of transducers, layout of the mine, on-site equipment, off-site equipment, local transportation systems, etc.

For a process such as manually swinging a sledge hammer, the signal will not be exactly periodic, i.e., there will be a slight variation in the time between hammer strikes. The signals can not be averaged by such a technique. However, if the same hammer strikes the same location and with the same force, the signatures—or shape of the waveform—produced will be closely repeated. It will be recognized that for hammer strikes, it is desirable that the same miner conduct all hammer strikes, as opposed to two miners interlacing strikes, i.e., like two railroad workers driving a spike in the old western movies. For signals generated by such mechanisms as a battering ram driven by a crew of miners, it would be less specific to an individual.

This property can be exploited by cross-correlating the signals for each individual cycle, or autocorrelating across multiple signatures. When the signals match, there will be a strong peak in the correlation between the signals. This will be true whether the signal is produced by a sledge hammer, a rock pounding on the mine floor, a backhoe pounding on the mine ceiling, a battering ram pounding on the mine wall, or whatever mechanism is used to produce the repeatable seismic vibration. The only requirement is that the same mechanism is used each time. In such a case, a signal generated at a first time is correlated with a similar signal generated at a second time, i.e., the signal generated at the first time replaces the “counterpart” signal of '124. The second signal can be thought of as a copy of the first signal delayed by the time between signals. In other words, by exploiting the symmetry between signals, the “counterpart” signal is not required to discriminate the signal from the background.

By knowing that the rescuers are looking for a signal within a prescribed time window (e.g. starting on the half hour for 2 minutes); that whatever the signal is, it will have the same signature; that it will have a known period, within a small time variation (e.g. repeating on 10 second intervals); makes the signal much more likely to be detected. It also builds confidence that there are no survivors in the absence of a received signal, which can also reduce the risk for rescuers that may otherwise risk early entry into an unstable mine.

By treating each event as a separate set of data, each set of data may be processed in combination with each other set of data. It is well known in the art that the number of combinations C of k events taken in combinations of r is

$\begin{matrix} {{C\left( {k,r} \right)} = {\frac{k!}{{r!}{\left( {k - r} \right)!}}.}} & (4) \end{matrix}$

For example, for a 2 minute window with events spaced by 10 seconds, k=12. If all possible pairs of events are cross-correlated, r=2, or

$\begin{matrix} {{C\left( {12,2} \right)} = {\frac{12!}{{2!}{(10)!}} = 66}} & (5) \end{matrix}$

which means that 12 events, such as hammer strikes, correspond to 66 combinations that can be used to extract a faint signal from the noise. If 2 transducers are used in an array, the number of combinations is 276. If 3 transducers are used in an array, the number of combinations is 630, etc.

It is well known in the art that the cross-correlation between two signals produces a peak corresponding to the time shift τ between the two signals. If a hammer is struck precisely on 10 second intervals, the cross-correlation between any pair of signals will produce a peak for τ=0 time shift. However, if one hammer strike is 100 ms out of sync with the 10 second spacing, the peak will occur at a corresponding τ=100 ms delay. Since it would be highly unlikely that a miner could control the timing to better than a second, the 66 cross-correlations would probably produce 66 peaks centered around τ=0 seconds, but have a distribution with a standard deviation of the order of 1 second.

It will be recognized that an impact will probably produce a ringing exponentially decaying vibration with a fundamental vibrational mode, or natural frequency, at frequency f₀, i.e., like ringing a bell. The natural frequency will depend on a number of unknown factors, such as: the materials properties of the rock and soil, the impacting device, the reproducability of the impacts, how the impact is coupled into the rock formation (striking an anchor bolt vs striking rock directly) and other factors. It will also be recognized that cross-correlation of two similar ringing signals will produce a maximum peak corresponding to τ=0, but will also produce secondary peaks corresponding to τ=1/f₀, 2/f₀, . . . m/f₀ where m is an integer. Thus the secondary peaks may be used to further detect the signature of a signal. In other words, the peaks of the correlation will be spaced in time in accordance with the delay between impacts, and secondary peaks will be characteristic of the natural frequency. It is well known in the art that driven signals will produce frequencies at the fundamental and higher harmonics instead of the natural frequency. Thus the signature of the correlation functions can be used to differentiate a driven signal from a natural frequency vibration.

If an array of receivers are used above ground, the signals can be cross-correlated between the receivers for the same events. In this situation, there may be a stronger correlation since the same signals would be cross-correlated. The time delay τ would be a function of the difference in transmission time between the source and the receivers, which would be a constant for each event. In this case, there would be a strong cross-correlation peak every 10 seconds or so during the up channel communication window which would disappear outside the up channel communication window.

It will be recognized that other sources of sound will be repeated also. For example, an air compressor, diesel engine, transformer humm, etc. will produce background noise that is also highly repeatable. Cross-correlation of those signals will also produce strong peaks centered around τ=0 seconds. If the load remained constant, they would have a much narrower standard deviation. For example, a bulldozer idling would have a very repeatable signal. The same bulldozer pushing dirt or navigating over uneven ground may not exhibit such a repeatable signal. Non-synchronous motors producing beat note signals will also repeat at the beat note frequency, which can have relatively long periods between repetitions. In some cases, it may be possible to identify and remove the repeatable background noise, including beat notes.

For array receivers cross-correlated for the same events, the background noise would produce peaks with a delay ‘r equal to the delay in the background noise traveling to the receivers. For example, if a transducer is located 2 ms from a noise source, and another transducer is 3 ms from the same noise source, there will be a strong cross-correlation between the two transducers with a τ of 1 ms. However, this peak in the cross-correlation will be a constant and easily ruled out as a signal from a miner. Moreover, this could be isolated in an engineering study and included in the processing software.

One possible modulation scheme could be to phase shift the hammer strikes within the 10 second window. This is similar to what is known in the art as Phase Shift Keying (PSK). For example, if the even hammer strikes are shifted by half the repetition rate, or 5 seconds, cross-correlation between the even and odd signatures would produce peaks at τ=5 second delays. For example 9:00:00, 9:00:15, 9:00:20, 9:00:35, 9:00:40 . . . . Such a scheme could be used to generate a simple binary (1,0) or (yes, no) reply to a question transmitted from above via the down channel. The repetition rate can be thought of as analogous to the baud rate in serial communication schemes such as RS-232 or RS-485.

Another possible modulation scheme could be to respond on the even times and skip the odd times for a yes, i.e., 9:00:00, 9:00:20, 9:00:40 . . . and respond on the odd times and skip the even times for a no, i.e., 9:00:10, 9:00:30, 9:00:50 . . . . It will be recognized that there should be a signal to indicate that an instruction or question was not clear or needs to be repeated and can not be answered yes or no. The possible encoding schemes for even such a simple mechanism are more than sufficient to convey the most important information. The exact scheme would have to be worked out and promulgated to all parties through training programs as part of the ERP.

The hardware requirements for the up channel from the miner end would be minimal. In the most elementary case, the timing could be derived from a simple watch. If there are two or more miners in a group, one miner could call out the timing, or cadence, to the miner producing the vibration, e.g., “on three, two, one, hit”.

A more advanced device would be a digital personal electronics device—much like a cell phone with a miner application software package, or a modified radio unit—with a display and input device. More sophisticated timing could be keyed in to produce the timing for a modulated signal. The screen could be color coded or modulated to count down the sequence, e.g., four seconds before the hit, start flashing a yellow screen every second for 3 seconds and a green screen on the second to hit. Alternatively, the device could have a speaker to deliver sounds, like a metrone, to count down the sequence.

For example, the coordinates of a location in the mine could be entered as text, and the personal electronics device could convert it to PSK sequences. At a higher level, the personal electronics device could compress the text into a more compact code requiring less information being communicated—much like MP3 compresses music files or JPEG compresses image files. Error detection could also be encoded by including something like a parity bit. The receiving electronics would then decode the encoded message back into the coordinates of the location. Such a scheme could compensate for the low bandwidth.

The power requirement for a timing device would be minimal. Moreover, the screen could be switched off when not in use, and automatically wake up at a prescribed time prior to the next communications window. Provisions could be made to allow the device to use power from a lantern battery as a backup. There would need to be a procedure to ensure the time is properly synchronized with a time standard. This could be automatically performed every time the miner is outside the mine—much like how the clock on a cell phone is synchronized automatically. Provisions could also be made to synchronize the clock with the start of communications from above ground, i.e., if the communications are set to start above ground at 2 minutes before the half hour, the personal electronics device could note the time and ask if the operator wanted to synchronize with the external clock.

All transducer signal data could be digitized and recorded. This would include trigger signals and “line” synchronization signals. The data could be processed on site, or the data could be sent to an off site laboratory for processing. For example, the Office of Mine Safety and Health at NIOSH could provide a central data processing facility to service the industry. The data could be sent via email attachments or other communications systems known in the art. The ERP, required to be on file with MSHA, could include the engineering study and maps showing the locations of the transducers to assist in processing the data. Results could be transmitted back to the site along with recommendations for ways to improve the S/N for the next communications window.

Turning to the down channel wherein information is communicated from above ground to below ground. The resources available at the transmitting end and the receiving end are asymmetric with the up channel, i.e., there are more resources available at the transmitting end, and fewer resources at the receiving end. The asymmetry in resources between the up channel and the down channel allows a modification for the down channel.

The most immediate difference is the availability of much better vibrational generating equipment—both in power and repeatability. For example, a vibrator, pile driver, jack hammer, low frequency pipe organ or resonant cavity (such as a steam whistle with one end burried in the earth), explosion, or other such device could be used to generate powerful seismic vibrations in a precise time sequence. Moreover, it will be recognized that an array of vibrational generators may be synchronized in an array to direct a vibrational beam in a particular direction, much like synthetic aperture radar. Similar to the predetermined window described for the up channel, the down channel could be prescheduled to take place at least 30 minutes after an accident and commence for a 2 minute window, 2 minutes before the half hour, i.e. preceding the up channel communication window. For example, if the accident occured at 8:15, the miners would know to expect to receive instructions at 8:58-9:00, 9:28-9:30, 9:58-10:00, etc.

The most elementary signals could probably be detected by direct sensory means such as by listening for sounds, or feeling vibrations. Simple detectors could be constructed by watching for vibrations in a cup of water or watching for a weight suspended from the ceiling by a shoe lace to swing in sympathetic vibrations. The bandwidth of such communications would be limited somewhat, but it would be a quantum improvement over no communications!

Modern handheld devices, such as cell phones, are now being equipped with sensors, powerful processors, and custom application software. For example, it is common for cell phones to include Micro-Electro-Mechanical System (MEMS) accelerometers and sophisticated signal processing capabilities. For example, US 2010/0048241 to Seguin et al., US 2010/0256947 to Kim et al., US 2011/0087454 to Lee et al., all three of which are incorporated by reference herein, disclose cellular phones with accelerometers and software configured to detect tapping on the phone as instructions for the phone operations. It will be recognized that such devices could be modified to receive seismic signals such as Morse code.

It would be reasonable to provide every miner with a handheld personal electronic device that could sense vibrations and decode the vibrations into text messages. The same device could incorporate the timing functions for the up channel as described hereinabove. It will also be recognized that such a device could be incorporated into a self-rescuer respirator package, hardhat, light, leaky feeder radio unit, or other equipment which is carried by every miner. Since any one of the devices could independently perform the functions, there would be inherent redundancy in the system, based on the fact that most accidents result in groups instead of individual survivors.

It will be recognized that in order to detect low amplitude vibrations, the device would be placed in contact with the mine wall, floor, or ceiling. In a typical scenario, a clock in the device could alert the miner at 8:56 that a communications window is to begin at 8:58. The miner would place the device on the mine floor or attach it to the mine ceiling and wait for a message. Starting at 8:58, a series of modulated vibrations would be generated above ground which would be received and decoded by the device. A text message would be displayed and recorded which could give the miners status information and detailed instructions.

The instructions could include modifications to the standard operation procedures, or ask simple yes/no questions. By default, from 9:00 to 9:02, the miners would respond to the questions or signal that they are alive, indicate their location, etc. If there was a question as to how many groups of miners there are, the down channel could instruct members of certain groups to respond in a certain order. For example, a prompt could be sent to the group with the highest seniority missing miner to respond first and proceed down the list by seniority number (much like a roll call) until a response is received. The procedure could also go by level in the mine, north to south, east to west, etc.

For example, if employee numbers 78, 157, 215, and 412 were missing, the message “78” could be sent via the down channel. If employee 78 could respond, he/she would respond by hammer strikes on 10 second intervals starting on the next minute mark. If a response is detected, the message “*” could be sent to acknowledge detection. The procedure would them be repeated to find employee 157, etc. After initial communications are established, a series of questions could be sent to establish which employees are in particular groups, and the group locations. A plurality of windows may be established—one for each group. For example, group one may signal from 9:00-9:02, and another from 9:02-9:04, etc. The details could be handled in the training.

An example up channel communications system 300 may be implemented into an ERP as shown in FIG. 3. A training program 305 is conducted for all underground and rescue personnel. The training would include such topics as determining the communications windows following an accident. This determination must be made independently by the underground and rescue personnel, based on the approximate time of the accident. The length of the communication window and data encoding scheme must be clearly established as part of the ERP.

Following an accident 310 each party would independently determine the actual up channel communication window times 315. At the prescribed times, the underground personnel would generate a plurality of acoustic events 320 in accordance with the training program 305. The rescue personnel would prepare the above ground equipment and record the plurality of acoustic events 325. The rescue personnel would process the recorded signals 330 and decode the message 335 in accordance with the data encoding scheme in the training program 305.

An example down channel communications system 400 may be implemented into an ERP as shown in FIG. 4. The same training program 305 is conducted for all underground and rescue personnel. Following an accident 310 each party would independently determine the actual down channel communication window times 405. At the prescribed times, the above ground personnel would generate a plurality of acoustic events 410 in accordance with the training program 305. The underground personnel would prepare to detect the plurality of acoustic events 415 and decode 420 the message in accordance with the data encoding scheme in the training program 305.

It will be understood that example times are merely to illustrate the spirit of the invention. Moreover, once initial communications are firmly established, the procedures may be modified to fit the circumstances. For example, it may be more important to continue drilling than to shut down operations for communications every half hour. A signal could be transmitted via the down channel that the next scheduled communication is to take place at 14:58:00 instead of the next default window. This would be acknowledged via the up channel, and the miners could rest until the appointed time. Of course the complexity of the modifications would depend on the miners having personal electronic devices to decode higher bandwidth signals. In the absence of such devices, the training program would need to cover contingency plans for modifying the default procedures.

An example plan for compliance with post-accident two-way communications requirements 500 may be implemented as shown in FIG. 5. A training program would be developed 505 for review. This would include input from and review by management, labor, engineering, local emergency personnel, and government agencies. An engineering study of the mine site and equipment would be conducted 510. This would include such things as accurate maps of the mine that tie into bench marks above ground and GPS coordinates, determining maximum distances to every portion of the mine and optimum locations for transducers and signal generators such that a reasonable energy impact can be reliably detected within a communication window. A plan would be developed to reduce background noise during a communications window 515. This would include determining which equipment could be switched off and means for minimizing vibrations during communications windows for equipment that can not be safely switched off. Transducers would be provided 520 in accordance with the engineering study 510. Personnel would be provided with personal electronic devices to detect vibrations, encode signals, and decode signals 525. A training program would be conducted for all personnel 530. The formal Emergence Response Plan would be written 535 and tested 540. The ERP would be submitted to the MSHA 545 and implemented 550.

Having disclosed the invention and example embodiments of the method in sufficient detail, other applications and embodiments customized to the specific requirements of particular mines or circumstances embracing the spirit and scope of the method will become apparent to those skilled in the art. 

1. A method for acoustic communication through-the-earth with steps comprising: (a) establishing a first algorithm for determining a second time of day based at least in part on a first time of day, wherein the first time of day is marked by an unscheduled event; (b) establishing a second algorithm for determining a first differential time period; (c) establishing a third algorithm for determining a second differential time period; (d) establishing a fourth algorithm for acoustically encoding at least one binary bit of information, wherein the fourth algorithm depends at least in part on the first differential time period; (e) determining by a first party a first estimation of the first time of day, wherein the first party is situated underground; (f) determining by a second party a second estimation of the first time of day, wherein the second party is situated above ground; (g) determining by the first party the second time of day based at least in part on the first algorithm and the first estimation of the first time of day; (h) determining by the second party the second time of day based at least in part on the first algorithm and the second estimation of the first time of day, wherein the first party and the second party determine the second time of day independently; (i) generating by the first party a plurality of acoustic events which encode the at least one binary bit of information in accordance with the fourth algorithm, wherein each of the plurality of acoustic events produces substantially a same acoustic signature to produce a plurality of acoustic signatures, the plurality of acoustic signatures propagate through-the-earth, the plurality of acoustic signatures are spaced apart in time in accordance with the fourth algorithm, and the generating starts at the second time of day and ends the second differential time period later; (j) detecting the plurality of acoustic signatures by at least one transducer and recording a plurality of digital acoustic signatures produced by each of the at least one transducers from at least the second time of day until the second differential time period later; (k) digitally processing at least two of the plurality of digital acoustic signatures to produce at least one correlated signal; (l) determining the at least one binary bit of information based at least in part on the at least one correlated signal, the second time of day, the first differential time period, and the second differential time period; and (m) communicating the at least one binary bit of information to the second party.
 2. The method of claim 1 further comprising a step of determining, with an acceptable level of confidence, if at least one member of the first party survived the unscheduled event, based at least in part on the presence or absence of the at least one binary bit of information communicated to the second party.
 3. The method of claim 1 further comprising a step of reducing a background noise level in at least one of the plurality of digital acoustic signals from at least the second time of day until the second differential time period later.
 4. The method of claim 3 further comprising a step of altering a load on at least one motor or engine.
 5. The method of claim 3 further comprising a step of using a first trigger signal synchronized to an electrical power line to determine the phase of the electrical power and using the phase of the electrical power to remove at least a portion of the background noise in at least one of the plurality of digital acoustic signatures.
 6. The method of claim 3 further comprising a step of using a second trigger signal synchronized to a rotating machine element to determine the phase of the rotating machine element and using the phase of the rotating machine element to remove at least a portion of the background noise in at least one of the plurality of digital acoustic signatures.
 7. The method of claim 3 further comprising a step of detecting the plurality of acoustic signatures by at least one transducer and recording a plurality of digital acoustic signatures produced by each of the at least one transducers for a time prior to the second time of day or a time subsequent to the second differential time period after the second time of day.
 8. The method of claim 1, wherein step (a) further includes a step of determining the unscheduled event based on a mining accident.
 9. The method of claim 1, wherein step (d) further includes a step of modulating the first differential time period to encode the at least one binary bit of information.
 10. The method of claim 1, wherein step (i) further includes a step of generating the plurality of acoustic events using a hammer or battering ram powered by a human, wherein the acoustic signature has an exponential decay time of less than the first differential time period.
 11. The method of claim 1, wherein step (i) further includes a step of using a personal electronic device to encode the at least one binary bit of information.
 12. The method of claim 1, wherein step (j) further includes a step of detecting the plurality of acoustic signatures by a plurality of transducers and recording the plurality of digital acoustic signatures produced by the plurality of transducers from at least the second time of day until the second differential time period later.
 13. The method of claim 12, further including a step of digitally processing a first digital acoustic signature produced by a first transducer of the plurality of transducers and a second digital acoustic signature produced by a second transducer of the plurality of transducers to produce the at least one correlated signal.
 14. The method of claim 1, wherein step (l) further includes a step of determining the at least one binary bit of information at a remote laboratory.
 15. A method for acoustic communication through-the-earth with steps comprising: (a) establishing a first algorithm for determining a second time of day based at least in part on a first time of day, wherein the first time of day is marked by an unscheduled event; (b) establishing a second algorithm for determining a first differential time period; (c) establishing a third algorithm for determining a second differential time period; (d) establishing a fourth algorithm for acoustically encoding at least one binary bit of information, wherein the fourth algorithm depends at least in part on the first differential time period; (e) determining by a first party a first estimation of the first time of day, wherein the first party is situated underground; (f) determining by a second party a second estimation of the first time of day, wherein the second party is situated above ground; (g) determining by the first party the second time of day based at least in part on the first algorithm and the first estimation of the first time of day; (h) determining by the second party the second time of day based at least in part on the first algorithm and the second estimation of the first time of day, wherein the first party and the second party determine the second time of day independently; (i) generating by the second party a plurality of acoustic vibrations which encode the at least one binary bit of information in accordance with the fourth algorithm, wherein the plurality of acoustic vibrations propagate through-the-earth, the generating starts at the second time of day and ends the second differential time period later; (j) detecting the plurality of acoustic vibrations by a personal electronic device, wherein the personal electronic device determines the at least one binary bit of information; and (k) communicating the at least one binary bit of information to the first party.
 16. The method of claim 15 further comprising a step of automatically modifying an operating state of the personal electronic device prior to the second time of day.
 17. The method of claim 15 further comprising a step of taking an action by the first party based on the at least one binary bit of information.
 18. A method for compliance with post-accident two-way communications requirements of the Mine Improvement and Emergency Response Act with steps comprising: (a) establishing a first algorithm for determining a second time of day based at least in part on a first time of day, wherein the first time of day is marked by an unscheduled event; (b) establishing a second algorithm for determining a first differential time period; (c) establishing a third algorithm for determining a second differential time period; (d) establishing a fourth algorithm for acoustically encoding at least one binary bit of information, wherein the fourth algorithm depends at least in part on the first differential time period; (e) conducting an engineering study of a mine site to determine a first acoustic background noise level, wherein the first acoustic background noise level corresponds to normal operating conditions; (f) establishing a first plan to reduce the first acoustic background noise level to a second acoustic background noise level from at least the second time of day until the second differential time period later: (g) providing transducers to detect acoustic events and communicate with an above ground center; (h) providing underground personnel with personal electronic devices configured to detect acoustic waves; (i) providing acoustic generation equipment configured to acoustically encode the at least one binary bit of information in accordance with the fourth algorithm, wherein the acoustic generation equipment is configured to generate acoustic power sufficient to be detected by the personal electronic devices; (j) providing training for the underground personnel and rescue personnel, the training including the first algorithm, the second algorithm, the third algorithm, the fourth algorithm, and the first plan; (k) developing an Emergency Response Plan based at least in part on acoustic communications; (l) conducting a test of the Emergency Response Plan for simulated emergency communications through-the-earth; (m) submitting the Emergency Response Plan to the US Mine Safety and Health Administration, or its successor; and (n) implementing the Emergency Response Plan for the mine site. 